Yeah it definitely doesn't think you visited on the 29th.
One thing to mention is that Firefox does cache pages even between loads. I know this because I build websites and for testing I run a server that needs manually launching. I can view the testing site in Firefox and then restart the computer, load Firefox and it'll show the testing site (even though the server isn't running) from cache.
I suggest something like that happened on the 29th and you didn't actually load any new pages. I'll let Jeff comment but I think it's more likely that your browser just didn't request a page on that day than the server somehow not logging your activity.
I've also checked your activity log which shows nothing for the 29th. If you want your uptime to be accountable, make sure you do something that leaves a date trail (post, comment, vote, flag, etc).